If Romney were to endorse Rubio, when would his announcement have the greatest impact?

Rubio would probably want it sooner, rather than later. He’d want it well before the Iowa caucuses. That would give him time to fully tap into Romney’s network of support in New Hampshire, and his fundraising connections throughout the country. It would also give Rubio more time to campaign with Romney in New Hampshire.

As an added bonus, Rubio must hope to get the same kind of all-in support from Romney’s ally, former Massachusetts U.S. Sen. Scott Brown. Brown also has an impressive network of support throughout New Hampshire from his unsuccessful race against U.S. Sen. Jeanne Shaheen.

Romney might be reluctant to endorse before Iowa, wanting to wait and see if Cruz crushes Trump there… and not wanting to be the target of insults from Trump and Cruz for over 30 days. But Romney knows that he has the potential to do what other GOP leaders would love to do, but don’t have the power to effect: Stop Trump, and help nominate someone with a better chance to win the general election and better suited to be president of the United States.